FIG. 2 shows a cross-sectional structure diagram of a conventional power semiconductor module. In this power semiconductor module 200, silicon power semiconductor elements 25 are soldered to a copper base substrate 23 with a solder layer 24a therebetween, the copper base substrate 23 having an insulating layer 21 and a circuit pattern 22. Then, a lead frame 27 is soldered to this structure with a solder layer 26b therebetween, and an external connection terminal 28 is connected thereto.
The number of silicon power semiconductor elements 25 mounted in the power semiconductor module 200 is determined based on the volume of the power semiconductor module 200. The silicon power semiconductor elements 25 are attached to the copper base substrate 23, the size of which is determined in accordance with this volume.
Then, a case 29 is attached to this structure, and a joint portion between the case 29 and the copper base substrate 23 is sealed with an adhesive (not shown). Subsequently, a sealing material layer 30 is placed in this structure. Silicone gel, a two-pack mixing type reaction material, is used as the sealing material layer 30. A predetermined amount of silicone sealing material layer 30. A predetermined amount of silicone gel is measured, mixed/stirred, primarily degassed for 10 minutes in a 13.33 Pa (0.1 Torr) vacuum environment, and then poured into the case 29. The resultant material is secondarily degassed for 10 minutes in a 13.33 Pa vacuum environment, and heated and cured at 120° C. for two hours, and then a lid 31 is placed onto the case 29, thereby completing the power semiconductor module 200.
When used, the power semiconductor module 200 is attached to a cooling fin applied with a thermal conduction paste.
When operating the power semiconductor module 200, it is important to transfer heat of the power semiconductor elements 25 from the copper base substrate 23 to the cooling fin via a thermal conduction paste 12, due to large current flowing through the power semiconductor elements 25 and the circuit pattern 22.
Patent Document 1: Japanese Patent Application Publication No. 2007-116172
Due to the better electrical characteristics of silicon carbide than silicon, the material of a power semiconductor element is expected to be changed from silicon to silicon carbide in the future. A power semiconductor element made of silicon carbide exerts its operating characteristics at high temperature which are better than the operating characteristics of a power semiconductor element made of silicon. Therefore, the power semiconductor element made of silicon carbide generates a high current density.
However, when a current of high density flows through a power semiconductor element, the amount of heat generated increases, resulting in an increase in the temperature of a sealing material in the vicinity of the element, the sealing material being used for sealing the power semiconductor element. When the power semiconductor element is made of silicon carbide in place of silicon, the temperature of the element reaches approximately 200° C. On the other hand, the temperature of an outer circumference of the power semiconductor module tends to become lower than that of the vicinity of the element.
Therefore it is important that the sealing material disposed in the vicinity of the element be resistant to heat, and it is also important to employ a sealing material that implements stable operation at high temperature.
A sealing material of a power semiconductor module having a silicon carbide element is added with aluminum hydroxide or the like as a flame retardant in order to deal with non-halogenation. Unfortunately, the problem in such a case is that the sealing material thermally deteriorates due to the flame retardant, lowering the thermal resistant performance of the power semiconductor module.